Until 1986, the phenomenon of superconductivity was limited to very low temperature ranges, and more particularly to the use of liquid helium. The record observed critical temperature was 23.3 K. for Nb.sub.3 Ge disposed in thin layers. In spite of this handicap, superconductive materials were expected to have numerous high-current applications, in particular in making electromagnets for obtaining intense magnetic fields. These possibilities were greatly reinforced by the discovery around 1970 of superconductive ternary chalcogenides which were studied, in particular, on account of their high critical field strengths.
During the last few years, considerable progress has been achieved in fundamental knowledge about solid-state physics, not only with respect to the interactions between superconductivity and magnetism in ternary chalcogenides, but also in relation to superconducting organic materials.
Mixed valence copper oxides derived from Perovskite have been studied for several years by the Crismat Laboratory at the University of Cannes in the context of new oxides having anisotropic metallic properties. These studies have shown that the preparation of such materials requires two conditions to be met concerning the nature of the element M constituting the lattice for receiving the oxide MO.sub.n :
the element M must be suitable for providing mixed valence, i.e. for providing two oxidation states simultaneously. The element is therefore a transition element suitable for electron delocalization in a band constituted from the dx.sup.2 -y.sup.2 and the dz.sup.2 atomic orbitals of the metallic element, and the 2p atomic orbitals of oxygen atoms by Linear Combination Atomic Orbitals (LCAO); and
in order to provide anisotropy it must be capable on its own of providing a mixed framework, i.e. of taking up several co-ordination numbers simultaneously. It is under these circumstances that copper was investigated as a possilbe candidate by virtue of its two possible ionization states: comprising the Cu.sup.3+ ion which is characterized by its aptitude for octahedral co-ordination numbers; and the Cu.sup.2+ ion for which the Jahn Teller effect allows octahedral, pyramid, and square co-ordination numbers.
A Perovskite framework is advantageous in making anisotropic structures because of its great simplicity, since it is formed solely by octahedrons interconnected at their vertices. In this way, it could be hoped that oxygen deficient Perovskites could be synthesized by creating anion defects in an ordered manner.
An article by Ninh Nguyen, Jacques Choisnet, Maryvonne Hervieu and Bernard Raveau of said Laboratoire de Caen, entitled "Oxygen defect K.sub.2 NiF.sub.4 -type oxides: The compound La.sub.2-x Sr.sub.x CuO.sub.4-(x/2)+.delta." which appeared in "Journal of Solid State Chemistry 39, 120-127, (1981)" is at the origin of this entire structural family. As a result, intergrowth then occurs between oxygen-deficient Perovskite sheets and insulating SrO type stoichiometric sheets. The oxygen-deficient sheets, characterized by simple Perovskite sheets which are isolated by SrO type layers, are genuine two-dimensional conductors in which the anion defects are situated in the base planes of the CuO.sub.6 octahedrons.
As described in an article entitled "Mixed Valence Ternary Copper Oxides of the Oxygen-Deficient K.sub.2 NiF.sub.4 Type: Progressive Evolution from a Conductive State to a Semi-metallic State of La.sub.2-x Sr.sub.x CuO.sub.4-(x/2)+.delta. Oxides" by N. Nguyen, F. Studer, and B. Raveau published in "Physs-chem. Solids" vol. 44 No. 5, pages 389-400, 1983, derived mixed valence ternary copper oxides of the K.sub.2 NiF.sub.4 type are characterized by a high defect density which extends over a wide range of electrical properties running from a semiconductor state to a semimetallic state, depending on the oxygen pressure to which they are subjected. The results obtained in this way, and previously obtained results for the following oxides: La.sub.3 Ba.sub.3 Cu.sub.6 O.sub.14+y and La.sub.2-x Sr.sub.x CuO.sub.4-(x/2)'.delta. show that such properties may be generalized to numerous ternary copper oxides characterized by a Perovskite derived structure either by creation of defects or else by intergrowth. The electrical properties of La.sub.2-x A.sub.x CuO.sub.4-x/2+.delta. oxides, in particular the properties of strontium compounds between 100 K. and 300 K. show that at low substitution rates (x&lt;0.3) the metallic conductivities at ordinary temperatures are two orders of magnitude greater than those of the oxide La.sub.2 CuO.sub.4.
While studying superconductivity in oxides having the composition BaLa.sub.5-x Cu.sub.5 O.sub.5(3-y) two Swiss physicists Bednorz and Muller demonstrated a restrictive transition at 30 K., with the temperature being reduced by applying high density measurement currents. In an article entitled "Possible High Tc Superconductivity in the Ba-La-Cu-O System" (Z. Phys. B, Condensed Matter, 64, 189-193, 1986), they consider oxygen-deficient metallic compounds in the Ba-La-Cu-O system, using the composition Ba.sub.x La.sub.5-x Cu.sub.5 O.sub.5(3-y) prepared in polycrystalline form. Samples having x=1 and x=0.75, with y&gt;0, and annealed at 900.degree. C. under reducing conditions, comprise three phases, one of which is a Perovskite type mixed valence copper compound. On being cooled, the samples demonstrate a linear decrease in resistivity, followed by a substantially logarithmic decrease which is interpreted as being the beginning of localization. Finally, a sudden decrease occurs through up to three orders of magnitude, indicative of a percolative superconductivity transition. The highest transition temperature observed was in the 30 K. range. The transition temperature is considerably reduced at high current densities. The authors believe that these properties thus result in part from the percolative nature, but may also result from 2D superconductive fluctuations in the double Perovskite layers of one of the phases present.
Superconducting oxides having a high critical temperature Tc have been discovered very recently in this way. The leading example is the oxide La.sub.2-x Sr.sub.x CuO.sub.4-y whose superconducting properties have been described by R. J. Cava, R. B. Van Dover, B. Batlogg, and E. A. Rietman in an article entitled "Bulk Superconductivity at 36 K. in La.sub.1.8 Sr.sub.0.2 CuO.sub.4 ("Physical Review Letters", volume 58, number 4, 26 January 1987). This article describes the result of resistivity measurements and magnetic susceptibility measurements for the compound La.sub.2-x Sr.sub.x CuO.sub.4 for x.ltoreq.0.3. The sample for x=0.2 has a superconductive transition at 36.2 K. over a width of 1.4 K. The associated diamagnetic susceptibility dc (Meisser effect) reaches a high fraction (60%-70%) of the ideal value. The authors estimate the density of states from the critical field and the resistivity values, and suggest, by an analogy with the compound BaPb.sub.-x Bi.sub.x O.sub.3, that conventional superconductivity is capable of explaining the high critical temperature in this class of materials.
The object of the present invention is to improve these characteristics, in particular the critical temperature Tc and/or the critical current, and also the sintering temperature.